The M2 protein is found in the viral envelope of influenza A virus and functions as a highly selective, pH-regulated proton channel important for the life cycle of the virus. Unlike neuraminidase inhibitors, rimantadine and amantadine are anti-viral agents capable of blocking the tetrameric M2 channel. In 2006, the CDC issued an alert instructing clinicians to avoid using M2 ion-channel inhibitors during influenza season due to the extraordinarily high frequency of amantadine resistance in influenza A isolates associated with a single point mutation in the M2 protein, S31N (Hayden F. G., Antiviral Resistance in Influenza Viruses—Implications for Management and Pandemic Response, N Enj J Med, 2006, 354;8). The drug-binding site is lined by residues that are mutated in amantadine-resistant viruses. Grambas, S., Bennett, M. S. & Hay, A. J. Influence of amantadine resistance mutations on the pH regulatory function of the M2 protein of influenza A viruses. Virology 191, 541-549 (1992); Bright, R. A., Shay, D. K, Shu, B., Cox, N. J. & Klimov, A. I. Adamantane resistance among influenza A viruses isolated early during the 2005-2006 influenza season in the United States. J. Am. Med. Assoc. 295, 891-894 (2006). Recently, it has been reported that resistance to rimantadine and amantadine in humans, birds and pigs has reached more than 90%, casting into doubt the continued ability of these drugs alone to satisfy the need for treatment of influenza (Deyde, V. M. et al. Surveillance of resistance to adamantanes among influenza A(H3N2) and A(H1N1) viruses isolated worldwide. J. Infect. Dis. 196, 249-257 (2007)).
Previous studies have suggested that BL-1743 (3-(4,5-Dihydro-1H-imidazol-2-yl)-3-aza-spiro[5.5]undecane) interacts differently with the M2 proton channel as compared with amantadine, but have found that the majority of isolated influenza viruses that are amantadine-resistant are also resistant to BL-1743. Tu Q, et al., Characterization of inhibition of M2 ion channel activity by BL-1743, an inhibitor of influenza A virus, J Virol. 1996 July;70(7):4246-52. For example, Tu Q, et al. found that mutations known to confer amantadine resistance at M2 residues 27, 30, 31, and 34, all within the M2 transmembrane domain, also induce “complete” resistance to BL-1743. Id. The publication by Tu Q, et al. concluded that “the overlapping spectra of amantadine and BL-1743 resistance mutations and the higher apparent Ki . . . do not indicate that BL-1743 should replace the use of amantadine (or rimantadine) for the prophylaxis or treatment of influenza virus infections in humans.” Id. See also Kurtz, et al., Growth impairment resulting from expression of influenza virus M2 protein in Saccharomyces cerevisiae: identification of a novel inhibitor of influenza virus. Antimicrob Agents Chemother. 1995 October;39(10):2204-9 (“BL-1743 does not produce an additive effect on M2 inhibition, suggesting that these two compounds interact with similar sites in the M2 protein . . . Thus, BL-1743 appears to represent a novel structure with an antiviral profile similar to that of amantadine.”).